The present invention relates to photographic color printing which requires a light source whose component color intensities are precisely controlled, and it is known that selective color modulaion of light may be accomplished with color absorption filters. Accurately calibrated color absorption filters are available in glass, acetate and gelatin sheets, but these all present problems in use in manners previously proposed. Handling accidents tend to break glass filters as does overheating. Acetate and gelatin filters scratch easily and heat tends to cause them to warp and buckle. Each surface of such a filter causes a minimum reflection light loss of about 5%. Soiling, scratching and buckling can increase such light losses to as much as about 40%, which destroys calibration precision and this is particularly troublesome when a number of such filters are used together to obtain a required color balance. Either packs of a half dozen or more filters must be used and tediously manually manipulated or a very large stock of filters must be availabe for slow and laborious selections therefrom.
It is a general object of the present invention to avoid or substantially reduce such difficulties in a simple and economical manner which assures obtainment of reliable results not heretofore readily attainable.
The present invention may feature the provision in photographic color recording devices such as optical printers of the type having a plurality of projector heads, e.g., horizontally axially aligned aerial image and master projector heads as taught in the United States Oxberry Pat. No. 3,040,619 of June 26, 1962; or in photographic color enlargers of well-known types wherein there are vertically aligned, in successive order downward, the lamp, negative holder, lens board and lens, and the usual bottom easel; of a cell containing a plurality of movable color absorption filters selectively manipulative into and out of alignment with the photographic beam of the apparatus and maintained submerged in a transparent and colorless liquid having an index of refraction substantially equal to that of the material of the filters. Reflection and refraction at surfaces are substantially eliminated and heat generated by the absorption of light energy is efficiently dissipated. The number of filters to be used together is limited only by function and space design. When this cell is constructed as a housing having a liquid-tight chamber as a part or the whole of the interior thereof in which the filters-submerging liquid body is confined for maintaining such submergence and that of the inner faces of the opposed windows and all other interior optical means, a reasonably large number of filters may be used therein with no apreciable deterioration in the efficiency of the system. Further, a desired filter value may be attained by a combination of smaller basic filters together so that the sums of the color densities thereof will substantially equal the desired amount. Through a judicious choice of basic filters one may combine them in various ways to obtain a wide range of component color intensities. The desired results may be consistently repeatable since the filters do no materially deteriorate.
Apparatus to perform these valuable functions in a relatively simple and easily realized and reliable manner may include a housing defining a plural-section chamber that has opposed and spaced sidewalls with the latter in an operational section of the chamber being provided with opposite windows closed by suitable optical transparencies through which the light beam path of a photographic color recording device may extend when this housing is supported in a beam-intercepting manner. A stack of planar color absorption filters, having substantially uniform indices of refraction, are arranged in three separate groups within a storage section of the chamber that is offset from the windowed operational section with those in one group being of the color cyan, those in a second group being of the color magenta and those in the third group being of the color yellow. Cyan modulates or controls how much of the red rays pass through, (2) magenta controls how much of the green rays pass through, and (3) yellow controls how much of the blue rays pass through; and thus they may be considered respectively to be, in terms of function, (1) white minus red, (2) white minus green, and (3) white minus blue. Manipulative means are provided which are capable of moving individually selectively each of these filters from the stack in the chamber storage section to positions of stacking between the windows and back again into the storage section.
At least the sides and bottoms of the entire chamber including its storage and operational sections are sealed in a liquid-tight manner. A body of transparent colorless liquid having a refractive index substantially equal to that of the filters is located in the chamber and its sections in a quantity to maintain immersion therein of the filters in their positions of storage and operation and movements therebetween, as well as the interior surfaces of the window transparencies.
Gelatin filters, which are preferred, are sheets of substantially pure gelatin except that they contain dyes which absorb specific color components of light. These have a specific index of refraction and are available in precisely calibrated color correcting form with color density multiples of 0.025. Consequently, the filtering capabilities of multiples thereof may vary from 0.025 to 1.575. The index of refraction of such gelatin filters is about 1.52 at the working temperatures thereof in apparatus of the present invention, and tetrachloroethylene has an index of refraction of about 1.51, which makes it ideal for use in practice of the present invention. Due to this substantial equality of these refractive indices there is no appreciable amount of light loss, if any, due to reflection in these apparatus, and no discernible directional disturbance of any light rays as they pass into, through, and out of an immersed gelatin filter sheet. Accordingly, if a clear gelatin filter sheet is kept clean by practice of the present invention it has no effect on the light beams passing through it due to the maintained immersion therein within the liquid bath.
In preferred forms of the present apparatus guide means are provided which define substantially parallel paths of planar motion of the filters between the chamber storage and opertional sections, and which also support them in substantially parallel positions intervening the windows. Manipulative means are provided for moving individually each of the filters along its path of planar motion between the storage section and the position intervening the windows as dictated by its guide means.
Additional objects are to provide such supplemental equipment for standard photographic color recording equipment which is relatively simple in construction and operation and requires relatively low cost to manufacture and maintain with the latter and the operation being quite easy to realize. Also optical precision and the ability to obtain with assurance precise color modulation are simply attainable.